A conventional four-stroke Otto cycle engine may be understood to be fueled by drawing a mixture of air and fuel (e.g. gasoline) into a combustion chamber/cylinder of the engine during an intake stroke thereof, typically through a port of an intake manifold. This may produce a homogeneous charge of the air-fuel mixture within the combustion chamber/cylinder, kept particularly close to a stoichiometric air-fuel ratio, which may be ignited by an igniter (e.g. spark plug) near the top of a compression stroke of the engine.
The air-fuel ratio may be understood as the mass ratio of air to fuel mixture introduced into the combustion chamber of an engine for combustion. When gasoline is used as a fuel, the stoichiometric air-fuel ratio is generally understood to be approximately 14.7 to 1 or, stated another way, 14.7:1. In other words, with a stoichiometric air-fuel mixture of 14.7 to 1, the air and fuel are balanced such that exactly enough air is provided to completely burn all of the fuel. If there is less air than required to maintain the stoichiometric air-fuel mixture, then there will be excess fuel left over after combustion, which may be referred to as a rich air-fuel ratio. If there is more air than required to maintain the stoichiometric air-fuel mixture, then there will be excess oxygen left over after combustion, which may be referred to as a lean air-fuel ratio. Now, while the air-fuel ratio may be commonly referred to in the trade, it should be understood that the air-fuel ratio may be more technically understood to be an oxygen-fuel ratio given that combustion is an oxidation process, and oxygen from the air is the oxidizer. However, while nitrogen and other elements in the air may not participate directly in combustion, such may have an effect on the oxidation rate.
Both a rich mixture and a lean mixture may present certain problems. Rich mixtures may produce cooler combustion gas than a stoichiometric mixture, however may have poorer fuel efficiency and increased pollution in the form of unburned hydrocarbons and carbon monoxide which may not be completely removed by the catalytic converter. Slightly lean mixtures may produce less power than the stoichiometric mixture, cooler combustion gas and increased pollution in the form of nitrogen oxides. Thus, it may often desirable to operate an internal combustion engine close to the stoichiometric air-fuel mixture. However, because a stoichiometric mixture burns relatively hot, it may damage engine components if the engine is placed under high load at this fuel air mixture. Consequently, for acceleration and high load conditions, a richer mixture (lower air-fuel ratio) may be used to produce cooler combustion products.
A homogeneous charge may be understood to provide stable combustion at a stoichiometric air-fuel ratio, but place limits on the engine's efficiency in running a lean mixture. Running at a lean mixture with a homogeneous charge may result in unstable combustion, which may lead to decreases in power and increases in hydrocarbon and carbon monoxide emissions.
In order to increase efficiency and power, fuel may be directly injected into the combustion chamber/cylinder (also known as direct fuel injection or “DFI”), as opposed to being injected from an intake port of an intake manifold (also known as port fuel injection or “PFI”).
Direct fuel injection may be used to provide a homogeneous air-fuel mixture within the combustion chamber/cylinder of the engine. Direct fuel injection may also be used to provide a stratified air-fuel charge, which may be referred to as fuel stratified fuel injection or “SFI”. In contrast to port fuel injection, direct fuel injection may direct fuel towards the igniter, rather than elsewhere in the combustion chamber/cylinder, which may provide a stratified charge. In other words, an air-fuel charge in which the air-fuel ratio is not homogeneous throughout the combustion chamber/cylinder, but varies across a volume of the combustion chamber/cylinder with distance from the ignitor.
Fuel injection timing may be understood to influence whether a direct injected air-fuel charge is homogeneous or stratified. For example, a homogeneous air-fuel charge may result when fuel is injected into the combustion chamber/cylinder of the engine during the intake stroke, while a stratified air-fuel charge may result when fuel is injected into the combustion chamber/cylinder of the engine during the compression stroke just before ignition.
Whether the internal combustion engine is configured to operate with a homogeneous air-fuel charge or a stratified air-fuel charge may be influenced by the operating load placed on the engine. For example, at low loads (e.g. constant or reducing speed with no acceleration), the engine may be operated with a stratified air-fuel charge, which may result in a lean burn and increased fuel economy. Alternatively, at moderate to high loads, the engine may be operated with a homogeneous air-fuel charge at near stoichiometric or slightly richer conditions.
When an Otto (e.g. gasoline) internal combustion engine is operated with a homogeneous air-fuel charge or a stratified air-fuel charge, such may be referred to as homogeneous charge spark ignition and stratified charge spark ignition, respectively.
A diesel internal combustion engine, on the other hand, may be understood to operate by stratified charge compression ignition (no ignitor or spark), where diesel fuel is injected into hot compressed (pressurized) air in the combustion chamber/cylinder of the engine during the compression stroke at the moment ignition is desired and self-ignites immediately.